Because the distance between text and reader shortened after the arrival of the printing press. In Roman and pre Gutenberg eras, text was inscribed, eg on monu

Because the distance between text and reader shortened after the arrival of the printing press. In Roman and pre Gutenberg eras, text was inscribed, eg on monuments. The reader might be a casual gawker, or someone at the base of a tower, or generally someone who could not be assumed to be in a position to engage closely with the text. Once print became widespread, that changed and the superior readability of mixed case won out- it was faster and easier to read. All caps was reserved for things that had to be consumable at speed, at distance and in short chunks, like newspaper headlines and mass media posters. Over time the use of upper case became synonymous with IMPORTANT and since shouting people inherently think their message is important, they used upper case too.